l. Field of the Invention
This invention relates to aqueous fluorocarbon polymer coating compositions and articles coated therewith and more particularly to such compositions and metallic articles coated therewith having improved resistance to degradation by hot oil.
2. Prior Art
In recent years, the use of fluorocarbon polymer coatings as non-stick finishes for metal substrates, particularly for cookware, has become widespread. The physical nature of fluorocarbon polymers make it difficult to bond them to metallic substrates sufficiently well to prevent coatings of the polymers from blistering and peeling during use. This is especially true of polytetrafluoroethylene.
Adhesion of such coatings has been improved by the addition of a water-soluble alkali metal silicate or a colloidal silica to the fluorocarbon polymer coating composition, as described in U.S. Pat. No. 2,825,664, issued Mar. 4, 1958 to James R. Huntsberger. In U.S. patent application Ser. No. 405,798 filed Oct. 12, 1973, adhesion of such coatings was improved by adding colloidal silica stabilized with sodium ions to the fluorocarbon polymer coating composition.
While the compositions described in the aforesaid references do improve adhesion to unprimed metal substrates, there are some end-use applications where resistance to degradation by hot oil is desirable. In cookware, for example, resistance to degradation by hot oil, especially at about 200.degree. C, is desirable in such articles as frypans, meat grills and sauce pans, where fats and oils are in contact with the fluorocarbon polymer coating during the cooking process.